The present invention relates to a powder molding press particularly utilized for a powder metallurgy technology.
In a conventional art, there has been provided various powder molding presses, one example of which is shown in FIG. 19 of a kind disclosed in the Japanese Patent Laid-open Publication No. 53-80867. Referring to FIG. 19, a powder molding press comprises a die plate 100 to which is mounted a die 101 provided with a die hole 110. Powder 111 filling the die hole 110 is pressed by the cooperation of upper and lower punch devices 104 and 105. The lower punch device 105 comprises first, second and third punches 105A, B and 105C disposed axially concentrically and relatively movably, thereby being capable of molding a stepped product.
First, second and third punch plates 107A, 107B and 108 are disposed with axially predetermined spaces with each other so as to correspond to the first, second and third punches 105A, 105B and 105C, respectively. The first and second punch plates 107A and 107B are movable and the third punch plate 108 is stationary.
The first punch 105A, which is the outermost one of the lower punch device 105, is secured be the first punch plate 107A disposed near the die plate 100 through a hollow cylindrical punch adapter 102A. The second punch 105B, which is disposed inside the first punch 105A, is secured to the second punch plate 107B disposed below the first punch plate 107A through a second punch adapter 102B which is inserted into the first punch adapter 102A so as to be axially movable along the inner peripheral surface thereof. The third punch 105C, which is disposed further inside the second punch 105B, is secured to the third punch plate 108 through a third punch adapter 102C disposed inside second punch adapter 102B.
In a molding operation, the upper punch device 104 is lowered by an upper ram, not shown, to be inserted into the die hole 110, and the die plate 100 is then lowered with a predetermined timing by a lower ram, not shown through a drawing yoke 106, whereby the powder filling in the die hole 110 is pressed into a molded product having a predetermined shape by the cooperation of the upper and lower punch devices 104 and 105.
The lowering movement of the first punch plate 107A is limited by a first mechanical stopper 109A disposed between the movable first and stationary third punch plates 107A and 108, and the lowering movement of the second punch plate 107B is limited by a second mechanical stopper 109B disposed between the movable second and stationary third punch plates 1078 and 108, whereby the first and second punches 105A and 105B of the lower punch device 105 are positioned in the die hole 110.
The first and second mechanical stoppers 109A and 109B are both secured to the stationary third punch plate 108 in an arrangement offset in angles of 90.degree. in the circumferential direction. Respectively two first and second mechanical stoppers 109A and 109B are arranged at symmetrical portions in cross shape, and in FIG. 18, the first and second mechanical stoppers 109A and 109B are shown in half sections with bilateral angular displacement of 90.degree. for showing them on one drawing sheet.
The conventional powder molding press of the structure described above, however, provides the following problems.
According to the structure of the conventional powder molding press, when the lowering movements of the first and second punch plates 107A and 10713 are limited by the first and second mechanical stoppers 109A and 109B, load acting points X1 and X2 of the first and second punch adapters 109A and 109B with respect to the first and second punch plates 107A and 107B and load supporting points Y1 and Y2 of the first and second mechanical stoppers 109A and 109B are offset in a direction normal to the axial direction of the mechanical stoppers by amounts of d1 and d2, respectively, as shown in FIG. 19. Because of the presence of these offsets d1 and d2, bending moments M1 and M2, shown by arrows, for downwardly bending the inner end of the first and second punch plates 107A and 107B are caused, which may result in the deformation of the first and second punch plates 107A and 107B, which may further adversely result in the displacement in position of the first and second punches 105A and 105B of the lower punch device 105 from the predetermined positions in the die hole 110, thus degrading the working performance of the molded product.
In order to obviate this defect, it may be possible to increase the bending rigidity of the first and second punch plates 107A and 107B by increasing the thicknesses thereof, which, however, results in the increasing of the total height of the molding press and also of the manufacturing cost.
In addition, since the first and second mechanical stoppers 109A and 109B are secured to the stationary punch plate 108 in cross shape, these mechanical stoppers are disposed at only two positions in a viewpoint of space, and accordingly, it is impossible to arrange more than two movable punch plates such as 107A and 107B.
Furthermore, it is necessary for the first and second punch plates 107A and 107B to be guided by guide rods, for example, not shown, for maintaining the relative positional relationship with respect to the upper and lower punch devices 104 and 105, but in a case where the bending moments M1 and M2 are caused, it becomes hard to maintain the relative positional precision between the die 101 and the upper and lower punch devices 104 and 105.